Method and apparatus for reflowing and annealing borophosphosilicate glass

ABSTRACT

A furnace with a large quartz tube with a gas inlet on one end, and a large opening at the other end for introducing and withdrawn wafers on a support. At least one small quartz tube is provided within and adjacent the sidewall of the large tube that has a gas inlet on one end outside of the large tube, and apertures in the sidewall. In use, a major stream of gas is provided by a gas inlet on the large tube that flows longitudinally through the large tube. A series of secondary streams of gas are provided from the apertures in the small tube that flow perpendicular to the major stream and across the surfaces of the wafers.

This is a divisional of application Ser. No. 08/289,650 filed Aug. 12,1994.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to integrated semiconductor devicefabrication and apparatus therefor. More specifically, the inventionrelates to improved annealing and reflow methods and apparatus forannealing and reflowing.

(2) Description of the Invention

Semiconductor devices are normally encapsulated in a glass or similarpassivation layer in order to chemically, mechanically and electricallyinsulate the enclosed array from the environments that the device willbe subjected to before, during and after the time the array or device istested and/or used for its chosen task. Glasses such asBorophosphosilicate glass (BPSG) is frequently used as a passivationlayer or interlayer dielectric film because (1) the BPSG layer has asubstantially lower reflow temperature than most other passivationmaterials, and (2) the presence of boron and phosphorus atoms inapproximately equal concentrations in the layer insures that suchmaterial will not produce a net N type or P type average carrier densityof substantial magnitude so as to cause inversion or other problems.However, BPSG has problems. BPSG is normally deposited, annealed andreflowed in a tube furnace made of quartz. The furnace consists of anelongated tube that is open at one end for introducing semiconductorwafers, and has a single gas inlet on the opposite end. The wafers arepositioned vertically, in spaced relation, on a suitable support andplaced in the furnace tube. A loose fitting enclosure is placed over theopen end and a heated gas introduced into the heated furnace through thegas inlet. The gas flows longitudinally down the length of the tubefurnace past the wafers which are positioned with their major or flatsurfaces perpendicular to the flow. With this apparatus, it has beenobserved that during reflow and annealing operations, BPO₄ crystals havea tendency to form by the re-crystallization of BPSG layer, particularlynear the center of the wafers. This leads to problems, more particularlyas the dimensions of the circuitry on the wafer are reduced in order toobtain greater miniaturization. The BPO₄ crystals may form about contactopenings 10, as illustrated in the cross sectional view of a device inFIG. 1. Since BPO₄ crystals can not be removed by either wet or dryetch, they will cause the poor contact opening and/or metal bridging.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved furnace forreflowing and annealing BPSG layers on substrates that prevents theformation of BPO₄ crystals in the layer.

Another object of the invention is to provide a new method of reflowingand annealing BPSG layers on semiconductor substrates wherein theformation of BPO₄ crystals in or on the BPSG layer is presented orminimized.

In accordance with the above objectives, there is provided an improvedfurnace for annealing and reflowing BPSG. The furnace has a large quartzfurnace tube with an opening at one end for introducing and withdrawingsemiconductor substrates into and out of the furnace. On the oppositeend is provided a main gas inlet. At least one small quartz tube extendsinto the furnace tube and is positioned adjacent the walls thereof. Thesmall tubes have small openings in the side wall that direct gas in adirection perpendicular to the longitudinal axes of the furnace tube.The tubes have the end in the furnace sealed. The opposite end has a gasinlet.

In the method of the invention a plurality of semiconductor wafers aremounted on a support in spaced relationship and introduced in a furnacechamber. The substrates are heated and a first stream of gas is flowedperpendicular to the wafer surfaces, and second streams of gases areflowed across the wafer surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a crossectional view of a typical integrated circuit,semiconductor device in which there is illustrated the problems relatingto conventional prior art BPSG technology.

FIG. 2 is a top view of a portion of FIG. 1 that illustrates prior artproblems.

FIG. 3 is a front view of a preferred embodiment of a tube furnace ofthe invention.

FIG. 4 is a crossectional view taken on line 4--4 of FIG. 3.

FIG. 5 is a top view of FIG. 3.

FIG. 6A, 6B and 6C are a front view, an end view of the open end, and anopposite end view, respectively, of another preferred specificembodiment of the furnace of the invention.

FIG. 7 is a crossectional detail view of a tube entry opening in thefurnace.

FIG. 8 is a crossectional detail view of a small tube with fitting forconnection to the entry opening.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawing, there is illustrated in FIGS. 3, 4, and 5,a preferred specific embodiment of the improved furnace of theinvention. The furnace 20 consists of an elongated furnace tube 22,typically made of fused quartz. Tube 22 has an open end 24 forintroducing and withdrawing semiconductor wafers for processing. On theopposite end, there is provided a main gas inlet 26 for introducinggasses into the tube 22. The semiconductor wafers 27, sometimes referredto as substrates, are in practice, supported on a suitable support 28referred to as a boot, for introduction into the furnace 20. As shown,the wafers 27 are positioned in spaced, upright position with thesurfaces being perpendicular to the longitudinal axis of the tube 22.Gas introduced into inlet 26 will pass longitudinally down the tube 22and out of opening 24, which is usually covered by a loose fittingcover, not shown. Two small tubes 30, normally quartz, are mounted intube 22 adjacent the wall. The tubes 30 enter the tube 22 throughopenings in spherical wall 32. The tubes 30 each have a plurality ofsmall openings 34 spaced along their length. The holes 34 are provided,at least, in the proximity of the wafers 27 when they are located in thetube 22. The tubes have an open end outside of the tube 22 which servesas a gas inlet 36. The opposite ends of the tubes 30 are sealed so thatgas escapes through holes 34.

The size of furnace 20 can be any suitable size to accommodate the sizeof wafers 27 to be processed. The wafers 27 have a typical diameter of100 to 200 mm. A furnace to accommodate the typical wafer has a furnacetube with an inside diameter of approximately 125 to 250 mm, and alength of approximately 150 to 250 cm. The small quartz tubes 30 have asdiameter of 10 mm, with the apertures spaced 20 mm, with diameters onthe order of 2 mm.

A more detailed furnace 40 is illustrated in FIGS. 6A, 6B and 6C.Furnace 40 has gas inlet 26 with an enlarged spherical end portion forconnection to a suitable source of gas. As shown, there is provided atotal of three small quartz tubes positioned on opposite sides andbottom of the furnace tube 22. A total of three inlets 44 for receivingthe small tubes are provided on spherical wall 46. The inlet 44 isillustrated more clearly in FIG. 7 of the drawing in layer scale. Theinlets 44 are fused to the spherical wall and are provided with aspherical end portion 48. The inside diameter of the protruding tubeportion of inlet 44 is greater than the outside diameter of the smalltubes 30. FIG. 8 depicts a fitting 50 that slips over tube 30 and isfused thereto. As shown, fitting 50 has a spherical end portion 52 thatmates with portion 48 on inlet 44. The spherical shapes of portions 48and 52 permit a minor adjustment of the position of the tubes 30 beforethe spherical portion are fused together.

In operation semiconductor wafers are placed on a suitable support andinserted within the furnace 20 or 40, as a unit through opening 24. Thewafers are then heated to a temperature in the range of 800° to 1000° C.The opening 24 is closed and heated gases are flowed into the furnacethrough inlets 26, and inlets 36 of tubes 22. The gas can be anysuitable single gas or a mixture of gases. In a reflow operation, atypical mixture consists of 90% N₂, and 10% O₂. When an anneal operationis performed, the gas can consist of 90% N₂ and 10% O₂. Other suitablemixtures are N₂ /O₂ /H₂, O₂ /H₂, or O₂ only. The wafers 27 aremaintained at a temperature in the range of 800° to 1000° C. by suitableheaters positioned around the furnace, indicated schematically as 50 inFIG. 5. In operation, the gases are flowed longitudinally along the tube22 past the wafers 27, from inlet 26. Gases from inlet 26 also flowbetween the wafers by turbulence. However, the central surface positionsare not subjected to sufficient flow by the gas from main inlet 26 toprevent formation of BPO₄ crystals. However, when gases aresimultaneously introduced into furnace through small tubes 30, gasescapes from holes 34 and is directed across the surfaces of the wafersand perpendicular from the gas flow from main inlet 26. The wafersurface therefore are exposed to sufficient gas flow to preventformation of BPO₄ and the deleterious effects of its formation. BPO₄growth is due to BPO₄ over saturate in wafer center area, that is causedby gas diffusion (flow) slowdown in center area. BPO₄ is thereforeeliminated or minimized by this invention.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. An improved furnace for semiconductor device fabrication comprising:a furnace tube formed of quartz having a large opening on one end to permit introduction and withdrawal of semiconductor wafers on a support; at least one small quartz tube in said furnace tube extending parallel to the longitudinal axis of the furnace tube and spaced therefrom and located on a side of said semiconductor wafers when said support is positioned in said furnace tube; said at least one small quartz tube having one open end for a gas inlet protruding outside of said furnace tube, and the opposite end being sealed by an end wall, said at least one small quartz tube mounted on said end wall with a swivel joint; said at least one small quartz tube having a plurality of small holes of a diameter less than the inside diameter of the small tubes located along the tube surface facing the central longitudinal axis of the furnace tube; a closure member for said large opening in said furnace tube, and a means to heat the interior of said furnace tube.
 2. The furnace of claim 1 wherein said furnace has a circular cross section.
 3. The furnace of claim 2 wherein said end wall on said opposite end is spherical shaped with said gas inlet centered in said wall.
 4. The furnace of claim 3 wherein said quartz tubes penetrate said spherical end wall and are positioned in close relation to said furnace tube.
 5. The furnace of claim 4 wherein said support for substrates is a boat provided with means to support the substrates in a spaced vertical position.
 6. The furnace of claim 1 wherein there are two quartz tubes within said furnace tube.
 7. The furnace of claim 1 wherein there are three quartz tubes within said furnace tube.
 8. The furnace of claim 1 wherein said furnace tube and small tubes are formed of fused quartz. 